1. Field of the Invention
This invention relates to analog amplifiers, and in particular to a fixed gain amplifier biased from two supply voltages.
2. Background Art
Analog integrated circuits are designed and fabricated in bipolar technology, in MOS technology, and in technologies that combine both types of devices in one process. The necessity of combining complex digital functions on the same integrated circuit with analog functions has resulted in increased use of digital MOS technologies for analog functions, particularly those functions such as analog-digital conversion required for interfaces between analog signals and digital systems.
When analog devices are implemented in the same technology as digital circuitry, the supply voltages used for the digital and the analog devices are the same. The selection of a supply voltage for a combined analog and digital circuit forces a tradeoff of the benefits of higher supply voltages against the benefits of lower supply voltages. In some cases, the semiconductor process used for a particular device may be chosen based on the associated supply voltage.
If the supply voltage is not sufficient for proper biasing and operation of the analog device, the analog components will induce noise into the analog signal. If the supply voltage is increased to properly bias the analog components, power consumption rises with the voltage increase.
In conventional designs, the analog devices are biased with a supply voltage sufficient for proper bias and operation. In the same circuit, the digital components are powered by a reduced voltage to reduce power consumption. The analog-digital conversion device receiving the analog input signal is powered from the analog supply voltage. Bridging the signal from the analog supply voltage to the digital supply voltage occurs after the analog to digital conversion is complete.
What is needed is a circuit and method for bridging an analog signal between two supply voltages to allow each analog device to use the supply voltage that provides minimum signal distortion and minimum power consumption.
The invention comprises a circuit and method for coupling analog signals between a circuit biased from a higher supply voltage to a circuit biased from a lower supply voltage. The invention comprises a transconductance amplifier coupled to an operational amplifier. The transconductance amplifier is biased using a higher supply voltage than the operational amplifier.
The transconductance amplifier receives an input voltage signal and converts it into an analog current. The analog current is coupled to the operational amplifier where it is amplified and output as an analog voltage representative of the input voltage signal.
Analog signals are bridged between devices biased with different supply voltages.
The higher voltage supply biases the transconductance amplifier for a more linear response.
The higher voltage supply also increases the transconductance amplifier signal to noise ratio.
The lower voltage supply biases the operational amplifier, reducing power consumption.
The invention is applicable to any two stage amplifier where first stage is biased from a high supply voltage and the higher current second (output) stage is biased from a lower voltage.
The invention allows reduced geometry devices to be implemented in the analog signal path.